The present invention relates to a fieldbus network and, more particularly, to a short circuit detector to protect a fieldbus network from an electrical short between the network's power/data wires and a shield connection.
In a typical industrial plant application, sensors measure position, motion, pressure, temperature, flow, and other parameters related to the operation of process machinery and activities. Actuators, such as valves and motor controllers, control the operation of the machinery and process activities. The sensors and actuators are remotely located from the human and computerized controllers which gather information from the sensors and direct operation of the actuators. A communication network links the controllers with the sensors and actuators located in the field.
Heretofore, communication between controllers, remote sensors, and actuators in industrial applications has been by means of analog signaling. The prevailing standard for analog networking of field devices and the control room in industrial applications has been the Instrument Society of America standard, ISA S50.1. This ISA standard provides for a two-wire connection between the controller and each field device. The wires carry power to the field device and, by the device varying the amount of current it draws, an analog signal representing some parameter, such as temperature or pressure. The signaling current is between 4 mA and 20 mA. The analog signal may be subsequently converted to a digital signal useful to a computerized controller. The wires also supply DC power for operation of the remote sensor or actuator.
Communication utilizing digital signaling reduces the susceptibility of the communication system to noise and provides a capability for conveying a wide range of information over the communication network. Digital communication also permits several different devices to communicate over a single pair of wires. Remote devices used in connection with a digital communication system typically incorporate local “intelligence.” This permits sensors and actuators to perform diagnostic, control, and maintenance functions locally. Further, the local intelligence permits the devices to communicate directly with each other and perform some functions without the necessity of involving a central control facility, thus promoting the development of distributed control systems.
Fieldbus is a generic term used to describe a digital, bidirectional, multidrop, serial communication network for connecting isolated field devices, such as controllers, actuators and sensors, in industrial applications. In general, a fieldbus is any two wire or two wire shielded line that is capable of carrying both power and signal to devices on a network. One such fieldbus is defined by the Instrument Society of America standard, ISA SP50.02 and International Electrotechnical Commission standard IEC 61158-2. These standards are utilized in the more detailed specifications for industrial networks promoted by Foundations Fieldbus and Profibus organizations. This system utilizes a two-wire bus to provide simultaneous digital communication and DC power to remotely located devices.
While fieldbus installations are as varied as the industrial applications with which they are used, an exemplary fieldbus installation is illustrated in FIG. 1. A twisted pair cable, comprised of a positive wire, 30, a negative wire 32 and a shield wire, 11, connects a digital control system 4 and a DC power supply 6 with a number of devices 8 (actuators, sensors, power supplies, and local controllers) in the field. The digital control system 4 and the DC power supply 6 may be located in a control room 10. The power supply 6 could be located in the field or at a marshaling panel. If wiring runs are long, it may be desirable to power the network from more than one point with additional power supplies 15. A power conditioner 22 is necessary to isolate the DC power supplies from the bus. The DC power supply will attempt to maintain a constant output voltage that, in the absence of isolation, would prevent propagation of the digital signal on the network. The shield is grounded at only one place along the cable. The outputs of the power supply are galvanically isolated from that ground. Hence the positive and negative wires of the cable only have a definite voltage between them and these voltages are not related to the ground. The development of the digital fieldbus may also mean that controllers are located in the field.
Several devices 8 can be connected to the home run 2 by spur cables 14 at a terminal referred to as a chicken foot 12 which incorporates signal termination for the home run. A terminator 16 comprising a resistor 18 and a series capacitor 20 connected across the wires of the home run cable 2 must be provided at both ends of the home run cable 2. The varying voltage of the digital signal is produced when an attached device varies the current drawn from the network producing a voltage drop across the resistor 18 of the terminator 16. The capacitor 20 of the terminator 16 prevents dissipation of the DC power through the terminator resistor 18 while permitting transmission of the high frequency digital signal on the bus. In addition, the terminators 16 serve to prevent signals from reflecting from the ends of the home run wires 2. The home run 2 is surrounded by a conductive sleeve or shield 11 which is connected to ground. The shield 11 helps to prevent RF noise from corrupting the data signal on the home run 2. A problem can occur, however, if either of the two wires of the home run 2 are short-circuited to the shield 11. The power supply leads have a given voltage differential between them but are not referenced to any other voltage or to the ground to which the cable shield 11 is connected. If either of the two wires of the twisted pair becomes coupled to the shield 11 through improper cable installation, deterioration of the cable such as a tear or a cut, or workman's error, the performance of the network can be substantially degraded.
First, since the network's power signal wires are a twisted pair, they are usually relatively immune to noise because they are balanced. Voltages induced by noise are present on both wires equally and thus appear as common mode voltage. Common mode voltage is relatively harmless because receivers in the devices connected to the cable are designed to reject it. If, however, one wire of the twisted pair becomes shorted to the shield, the two wires will no longer be balanced. Noise voltage would then affect one wire more than the other and would produce a differential noise. Differential noise is difficult, if not impossible, for receivers in the downstream devices to eliminate and therefore degrades the receiver's ability to detect signals correctly.
Secondly, a short between one of the cable wires and the shield can disable current limiting devices that are typically used between the trunk cable and the drop cable. Such devices are shown, for example, in U.S. Pat. No. 6,369,997 and in U.S. Pat. No. 6,366,437. These current limiting devices depend on the signal and power wires being galvanically isolated from the shield, that, is, not electrically conducting, to the shield. If they are not, a short circuit on the drop cable can bypass the protective current limiting circuit and cause the loss of power to the entire network.
Often when a failure occurs, its cause is not readily known. If an entire trunk cable loses power, the entire system is shut down until it can be determined whether the problem lies with the various components such as power supply and power conditioning circuits, devices connected to the trunk cable, or the trunk cable itself. This type of diagnostic work is costly and time-consuming.